The treadmill one type of exercise equipment suitable for exercise, as well as for electro cardiographic or other medical evaluation. A typical treadmill comprises an endless belt extending between two parallel spaced rollers. A support member typically underlies the upper span of the belt. An electric motor drives one of these rollers to advance the belt over the support at a selectable speed. DC motors are usually implemented in treadmills because they are suited to be operated over a wide range of speeds.
The DC motor is typically driven and controlled by a motor drive control circuit. A DC voltage is controllably applied across the armature of the DC motor to selectively establish the speed of the motor. In most forms of motor drive controls, current sensing of the motor, namely, the current being conducted through the armature of the motor, is sensed to ascertain and controllable establish and maintain the speed of the motor. It is necessary to maintain a controllable and constant belt speed, especially during variations of the load applied or removed from the motor, i.e., a person entering, using, and exiting the treadmill. It is important that the motor control be capable of responding quickly to transient loads. Therefore, it is important to sense the current through the armature of the DC motor, especially during transient conditions to compensate for the voltage drop across the armature of the DC motor. Noise from the associated DC motor and other circuitry is a concern when sensing motor current.
Referring to FIG. 1, there is shown a simplified version of a prior art DC motor drive control 10 wherein the motor (load) 12 is connected to the source terminal of a power switching device 14, shown in this embodiment as the source terminal of a field effect transistor (FET). A pulse width modulation (PWM) control circuit 16 is seen to drive the gate of the FET to control on/off time i.e. duty cycle, and is referenced to the source terminal of the FET as well, and thus, this configuration is known as a "common source connection". In this common source connection, the control circuit 16 "moves" up and down with the motor modulation. This undesirably results in "noise" paths such as via stray capacitance, radiation, etc. which can degrade performance and cause irregularities of the DC motor 12 depending upon the installation and surrounding environment. This common source connection further necessitates that any external control, such as a potentimotor varying the control 16 is "hot" and is subject to motor modulation.
Referring now to FIG. 2, there is generally shown at 20 an alternative prior art DC motor drive control circuit having a "common drain connection". In this embodiment, the source of a power switching FET 22 and a PWM control circuit 24 are at a common ground, i.e., the negative rail of the DC voltage source provided by the bridge. The PWM control circuit 24 in this configuration driving motor 26 is influenced by noise to a much lesser extent than that of circuit 10 shown in FIG. 1. If this configuration is employed in a control drive using bridge rectification, as shown, in a common scenario the "common" will be modulated with a rectified sign wave corresponding to the AC source frequency, usually 50 or 60 Hertz, which is a much less noise problem than the high frequency problem in FIG. 1. One such embodiment of this common drain connection in a DC motor drive control is depicted in U.S. Pat. No. 5,351,336 and U.S. Pat. No. 5,367,600. The disadvantage of this circuit is that current sensing is obtained at the drain level. The disadvantage of this circuit is that it requires an optocoupler and two separate isolated ground levels, a sensing ground and a power supply ground.
The control circuit 20 of FIG. 2, however, induces another problem. Referring to FIG. 3, there is depicted the control circuit 20 including the power switch 22 shown as S1, typically comprised of a FET, illustrating the voltage Vs produced at node N by the power switch S1, and the resulting motor voltage Vm. Notably, the motor voltage Vm is continuous, i.e., it does not go to zero when switch S1 is on in the short time frame at the high switching frequency. The motor voltage Vm in this embodiment is sustained by the back EMF of the motor 26 as represented by the armature motor current Im. The diode D1 is a free wheeling diode which provides a path for the current when the switch S1 is in the off position. Note, however, that the FET current Is is discontinuous and "pulses" at the modulation frequency. Given that the ideal location to sense the motor armature current is referenced to the common or negative rail, this drive circuit is problematic in that the current sensed in the FET source of S1 does not adequately represent armature motor current, but, rather represents "peak" motor current. This control circuit provides the advantage that the PWM control circuit 24 is referenced at the negative rail, but has a disadvantage in that there is no way to adequately sense armature motor current.
There is an desired an improved DC motor drive control circuit having a common drain connection further including the ability to effectively sense or replicate armature motor current at the FET source.